Now 3760@34xx shows suppor for 3D and DEM maps in GarminDevice.xml, but it doesn't work correctly. I get only DEM shading but no 3D terrain. There is no 3D buildings, only flat bases of 3D models appear.

ASR works after downloading *.srd files with WebUpdater. These are different files than for 37xx, here list form GarminDevice.xml

If you do not want/need to use a different firmware, just follow these steps to enable the 3790 options:

1- Find your current firmware for your GPS (gcd or rgn)
2- Open it in RGN_Tool
3- In Tools, run the 3790 patch
4- Save to rgn
5- Drag the new .rgn over updater.exe from Garmin
6- Put your GPS in service/pre-boot mode with USB cable
7- flash by clicking Ok in updater.exe

For more details about flashing and pre-boot mode, see Step 3 & 4 at the bottom of this thread:

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